The berries in my grape clusters are not ripening evenly. Why?

FAQ
Question: 

The berries in my grape clusters are not ripening evenly. Why?

Answer: 

Several factors could be responsible for the uneven ripening of the berries within a cluster.  Possible causes are over-cropping (too many grape clusters on the vine), a potassium deficiency, moisture stress, or 2,4-D damage. 

Over-cropping is the most common cause for home gardeners.  An average grapevine may have 200 to 300 buds which are capable of producing fruit.  If grapevines are not pruned properly in late winter, the number of fruit clusters may be excessive.  The vine is unable to ripen the large crop properly, resulting in uneven ripening of the berries within the clusters.  In Iowa, 60 is the maximum number of buds that should remain on a grapevine after pruning. 

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